1. Field of Invention
This invention relates to the field of data communication and, in particular, the field of RFID data communication.
2. Description of Related Art
The technology of locating radio-reflecting objects was developed in the 1940s and refined for many applications for military and commercial use. This technology is known as radar. Airplanes and weather are routinely tracked. Location is determined from a combination of delay (round trip time for the radio wave) and antenna pattern. The doppler shift of the radio wave provides the speed of the object that is being tracked. Pulsed and swept frequency methods have been used for radars as well as pulse compression techniques to improve resolution.
These conventional radar systems are not suitable for tracking a RFID tag in the midst of clutter. The tag is often the smallest radar target in the field of view. Thus, the techniques used for normal pulsed doppler or swept frequency radar systems are incapable of measuring the distance to a tag. Conventional radar systems also require a large bandwidth to achieve sub-meter accuracy. High bandwidth is not allowed in many countries for passive RFID systems.
Some tag-reader systems use a tag that contains an active radio transmitter. It is possible to measure the angle of arrival of the radio wave from the tag at several locations and draw vectors back toward the tag. The tag is located where the vectors intersect. Active transmitter tags are in use for transportation applications, but the cost, complexity, power requirements and size of the tags are barriers that limit their suitability for low-cost tagging applications. It is also possible for the tag to transmit a direct sequence spread spectrum signal that is received by multiple receivers. The received signals can be processed providing the location of the tag in space. (This same principle is used for the Global Positioning Satellite system but with multiple transmitters that transmit signals which are received by terrestrial GPS receivers and processed to provide the receiver's location). Tags using this principle are even more costly and complex than the simple transmitters of the first example. Another approach is for a tag to transmit an ultra wideband signal (UWB) and use readers to measure the angle of arrival of the first signal acquired. Thus, the effects of multipath are minimized. These types of tags contain a source of power for operation such as a battery and require high bandwidth, and thus the technique is not suitable for passive RFID tags. Tags may also measure the strength of signals transmitted by two or more readers and infer its position as being closest to the reader with the strongest signal. While this method may be useful for some applications, a precise location of the tag is not obtained. Two or more readers may measure the strength of signals from a tag and triangulate based on the relative strengths. This method requires reader hardware at two locations which increases complexity and cost, and the method is not accurate since the strength of signals produced by tags vary because of many factors and not only distance.
Backscatter tags modulate and reflect (or retransmit) the radio signal that is received by the tag antenna. The modulated backscattered signal can be distinguished from the unmodulated background signals produced by reflections from other objects in the field of the reader antenna. U.S. Pat. Nos. 4,075,632, 4,360,810 and 4,739,328 are included here by reference and are representative of the technology. All references cited herein are incorporated by reference. The signals produced by these tags can be used to measure the speed and direction of travel of a tag (Koelle, U.S. Pat. No. 5,510,795) in the presence of other reflecting objects. The identification number (or other information stored in the tag) is unambiguously linked to the speed and direction of travel of the tag measured by the reader.
Hane (U.S. Pat. Nos. 4,728,955, 4,804,961 and 4,851,851) has shown how to measure the distance and direction to tags that use a single sideband suppressed carrier modulation using a subcarrier (as described by Henoch, U.S. Pat. Nos. 4,358,765 and 4,333,078). The types of tags using this technique are complex and restricted in use to a limited frequency band. The modulation produced by the systems of Koelle, et. al. use direct modulation of a frequency-shift code on the radio frequency carrier and the methods of Hane are not suitable, nor can be extended to determining direction and distance to the tags of Koelle, et. al. The direction to backscatter tags can be measured using the in phase (I) and quadrature (Q) received signals (U.S. Pat. Nos. 6,476,756 and 6,600,443). The direction is linked to a specific tag and is obtained in the presence of other reflecting objects. The direction to the tag may be measured at several locations and the location of the tag inferred to be at the intersection of the lines of direction.
All references cited herein are incorporated herein by reference in their entireties.